1. Field of the Invention
The present invention relates to magnetic record apparatus, and more particularly to preamplifier means for a magnetic reproducing, or playback, head.
2. Description of the Prior Art
It is, of course, the objective of any record playback system to faithfully reproduce the signals which were impressed on the recording medium, free from distortions and superimposed spurious signals known as noise. In the art of magnetic recording, it has been found that much of the noise experienced in the reproduction of the recorded signals was due, in part, to the magnetic coating on the tape, and, in part, to the efficiency of the reproduce head. Those problems have been challenged and the technology has now advanced to the point where the limiting factor in the signal to noise ratio of the reproduced signal is in the electronic amplification system. Heretofore, because of the peculiarities of transistor circuitry, efforts to improve the signal to noise ratio on wide band (up to 2 MHz) systems have encountered problems.
Efforts to overcome the noise factor in the lower frequency range have usually required that the input stage of a preamplifier be operated at a relatively high current. The input impedance in the transistor amplifier is a function of the internal resistance of the transistor as is the internal noise generator (E.sub.n) thereof and that internal resistance is inversely proportional to the current.
When a low internal resistance and low internal noise generator (E.sub.n) is required, such as when the magnetic transducer has a low impedance and, therefore, an intrinsically low thermal noise contribution, a conflict arises at the higher frequency range. At the higher frequency range, the reproduce head, or transducer, being an inductive device, exhibits a much higher impedance. The lower input impedance of the amplifier tends to load the transducer at the higher frequencies. Further, the high current in the input stage also tends to maximize the base emitter capacitance of the transistor; that will tend to lower the resonant frequency of the transducer to within the desired 2 MHz pass band, cause problems in amplitude and phase equalization and, further, cause the noise match of the preamplifier to the transducer to be non-optimum due to a component of noise caused by the noise current of the input stage of the amplifier flowing through the impedance of the transducer (I.sub.n Z), the transducer impedance being maximized by the high input capacitance. Known efforts to overcome such undesirable effects by using degenerative feedback tend to increase the noise level for the lower frequency range by increasing the internal resistance of the transistor or cause instability at the higher frequency range due to the high Q of the transducer.